The present invention pertains to a procedure for manufacturing transparent, impact-resistant styrene resin, which procedure comprises anionically copolymerizing a conjugated diene and a vinyl aromatic hydrocarbon monomer in a concentrated homogeneous medium in an aromatic hydrocarbon solvent.
Anionic copolymerization for the manufacture of such products is known and described in numerous patents, one of the most typical of which is U.S. Pat. No. 3,639,517. According to this patent, the anionic copolymerization is carried out in multiple stages at pressures that are high enough to maintain the reaction medium in a highly liquid state. Although it is stated that the copolymerization may be carried out without solvent, it is impossible to carry out this anionic polymerization in dry phase on an industrial scale. In order to obtain a liquid medium suitable for anionic polymerization in dry phase, it is necessary to operate at temperatures higher than 180.degree. C. which very quickly and irreversibly degrade the active sites. It is therefore necessary for this process to employ reactors with very short residence times on the order of several tenths of seconds. This type of system is too risky from an industrial point of view. It is clear from this patent that in practice anionic copolymerization of vinyl aromatic monomers and conjugated dienes must be carried out in a highly diluted medium. In addition to the drawback of handling a large amount of solvent which requires voluminous apparatus and additional operations for final elimination of the solvent, anionic copolymerization in a highy dilute medium induces a more extensive degradation of the active sites of the polymeric masses than in the case of copolymerization in a concentrated medium. Nevertheless, none of the described procedures has demonstrated the possibility of carrying out anionic copolymerization in a concentrated medium. The known maximum concentrations are on the order of 30% by weight of polymerizate in the solvent. In order to maintain a liquid, homogeneous reaction medium, this concentration cannot effectively be exceeded in most of the known solvents employed in this type of anionic polymerization. Higher concentrations lead to a hetrogeneous liquid medium which manifests itself by the presence of multiple phases. These multiple phases induce in the final copolymer a hetrogeneity of structure, e.g., of molecular weights and index of polydispersity, which makes it very difficult or even impossible to control the final properties of the resin.